I need to downgrade my Mac Book Pro to MacOS BigSur

Which Mac? Use 🍎 > About This Mac, and post the model and model year. With both the model and model year, we can determine whether the Mac can run Big Sur.

There is no downgrade for a Mac, only erasing and re-installing, or erasing and restoring a backup from Big Sur—for a Mac that can run Big Sur. Apps and app files can get updated along with macOS upgrades, and which then makes downgrading a tediously manual process, at best.

Or if there’s enough storage available, installing a hypervisor and then installing Big Sur as a guest—again, if the Mac can run Big Sur.

If you have a MacBook Pro 2021 for instance, it can’t run Big Sur.

Erase all content and settings on Mac - Apple Support

How to reinstall macOS - Apple Support

Building a bootable installer containing Big Sur is among the the easiest paths:

How to create a bootable installer for macOS - Apple Support

Or you run whatever app requires Big Sur, and see if it works on Monterey.